(a). Field of the Invention
The present invention relates to an automatic transmission for vehicles. More particularly, the present invention relates to a shift control method for an automatic transmission which can prevent shifting retardation and increase in an RPM when a downshifting control is performed during the upshifting operation.
(b). Description of the Related Art
In the automatic transmission used for vehicles, a shift control system performs control to realize automatic shifting into different speeds and shift ranges according to various factors including throttle opening, vehicle speed and load, and several other engine and driving conditions detected through a plurality of sensors. That is, based on such various factors, the shift control system controls a plurality of solenoid valves of a hydraulic control system such that hydraulic flow in the hydraulic control system is controlled, ultimately resulting in shifting of the transmission into the various speeds and shift ranges.
In more detail, when the driver manipulates a shift lever to a particular shift range, a manual valve of the hydraulic control system undergoes port conversion as a result of the manual valve being indexed with the shift lever. Accordingly, hydraulic pressure supplied from a hydraulic pump selectively engages a plurality of friction elements of a gearshift mechanism according to the duty control of the solenoid valves, thereby realizing shifting into the desired shift range.
In such an automatic transmission, shift quality is determined by how smoothly the friction elements are engaged and disengaged. Namely, when changing shift ranges and especially when changing speeds in the drive D range, the timing between the engagement of a specific set of friction elements in relation to the disengagement of another specific set of friction elements determines the shift quality of the automatic transmission. Accordingly, there have been ongoing efforts to develop improved shift control methods that enhance shift quality by better controlling the timing of friction elements to engaged and disengaged states.
In such shift control methods developed to date, when the vehicle is shifting from a second speed to a third speed and a kickdown state is realized by a driver depressing a gas pedal as shown in FIG. 6, a kickdown signal cannot be output. That is, the kickdown signal is output only after the second-to-third upshifting operation is completed and the hydraulic pressure within a hydraulic circuit is stabilized.
That is, since a shift control unit cannot identify the stroke variation of each piston for friction elements operated during the second-to-third upshifting operation, it cannot accept the command to downshift during the upshifting operation.
In addition, since stroke variation of each piston for the friction elements cannot be pre-estimated during the upshifting operation, a plurality of one-way clutches for the respective speed ratios should be provided, increasing the weight of the vehicle and the manufacturing costs.